But if healthy adults are not able to live long enough reproduce, then the captive populations in Europe won't be self-sustaining—a problem that American zoos already face.

Survival Strategies

To keep zoo elephants alive longer, the authors recommend routine screening for obesity (something that's done in U.S. captive elephant populations), as well as monitoring stress via a chemical known as interleukin-6.

Checking this biological marker, which shows that the body's immune system is battling sickness, would allow zoo officials to intervene before the animal is seriously ill, Mason said.

Robert Wiese, collections director at the San Diego Zoo in California, was not part of this study. He said making a comparison between the lifespan of captive and wild elephants may seem deceptively simple.

"There are just so many confounding issues, especially in small sample sizes [of] zoo animals, that it's hard to really separate and make sure you're comparing apples to apples," he said.

Huge Improvements

In 2004 Wiese co-authored a paper in the journal Zoo Biology showing the opposite of Mason's findings: that zoo elephants live as long as those in the wild.

He said that within the last decade accredited facilities have made huge improvements in the care of captive elephants by providing better nutrition to combat obesity, as well as environmental enrichment activities that reduce stress.

"Typically anti-zoo critics throw in our face the things that the zoo community has already identified as issues that we need to work on," Wiese said.